Seismic surveys are conducted for the purpose of mapping subsurface geologic structures. The ability to image and map small subsurface structures depends on the bandwidth (the range of frequencies at which the amplitude of the signal exceeds that of the noise) of the signals recorded. Typical values of the bandwidth are 10 Hz to 60 Hz for a conventional seismic survey.
In seismic recording, the smallest vertical distance between two features of a structure that can be resolved is considered to be in the range of one half to one quarter of a wavelength. Therefore, to image smaller features, the signal bandwidth needs to be expanded to include smaller wavelengths or a larger band of frequencies. Consequently, seismic surveys with the goals of recording high-resolution data should be designed to increase the frequency of the signal as much as possible.
Although higher frequencies increase the ability to resolve smaller features, recording these frequencies present several problems. During the acquisition of high-resolution data, the high-frequency signals can be degraded by seismic equipment such as receiver instruments, recording arrays, and sources. Conditions at or near the surface, such as ambient noise, surface waves, and air waves can also interfere with the signals. In addition, high-frequency seismic recording is inherently limited by attenuation of the seismic waves as they travel through the earth. The high-frequency components suffer more loss of amplitude than the low-frequencies and therefore, they may not be distinguishable over ambient or surface noise.
As mentioned, the recording of high-frequency seismic signals is inherently limited by attenuation of the seismic waves as they travel through the earth. The amplitude (A) of a seismic wave of frequency f that propagates a distance z is given by EQU A=A.sub.o e.sup.-z/L
where A.sub.o is the initial amplitude of the seismic wave and where the attenuation length L (the distance in which the amplitude decreases to approximately one third of its original value) is given by ##EQU1## The attenuation length L is useful in evaluating the effects of attenuation. The parameters of L, in addition to the frequency, are the velocity of the seismic wave v in the earth and the attenuation quality factor for the earth Q (ratio of the power stored in a material to the power dissipated). The parameters va nd Q depend on the properties of the earth. Lower values of v or lower values of Q correspond to shorter attenuation lengths and quicker reduction of the high-frequency components of the seismic signals. By the time the seismic wave propagates a distance of several attentuation lengths (L), its amplitude will be greatly reduced and may fall below the noise level. This can result in the loss of the ability to detect and record smaller features.
At depths of exploration interest (generally 1,000 to 20,000 feet), values of v range from 8,000 to 20,000 feet/second and values of Q range from 50 to 300. Table 1 shows a calculation for the wavelengths .lambda. and attenuation lengths L for a number of different frequencies. Table 1 shows that the lower frequencies can be used to image large targets which are quite deep; however, use of the higher frequencies to image smaller targets is depth-limited. The signals at 80 Hz, that can be used to image targets with thickness of (.lambda./4) or 30 feet, are reduced to 1/3 of their original value by the time they travel 4,000 feet in the earth.
TABLE 1 ______________________________________ WAVE LENGTHS AND ATTENUATION LENGTHS AT DEPTH v = 10,000 feet/second Q = 100 f (Hz) .lambda. (ft) L (ft) ______________________________________ 10 1000 32000 20 500 16000 40 250 8000 80 125 4000 100 100 3200 ______________________________________
Seismic wave propagation through the unconsolidated soil between the surface and water table (which typically ranges from 0 feet to between 10 to 2,000 feet below the surface) can also attenuate the higher frequencies. This region is often called the weathered layer and varies widely from site to site. The weathered layer is known to have lower velocities and quality factors than deeper, more consolidated sediments. As stated earlier, lower values of v and lower values of Q correspond to shorter attenuation lengths and quicker reduction of the high-frequency components of the seismic signals.
Compressional wave velocities of the weathered layer are typically 1500-3500 feet/second. Only a few measurements of Q have been quoted in the literature and these range from 10-30. Attenuation lengths for the weathered layer are given in Table 2, and it can be seen that seismic wave propagation through the weathered layer reduces the amplitudes of the higher frequencies. For example, the amplitude of an 80 Hz signal is reduced to one-third of its original value after traveling through 160 feet of the weathered layer. The results in Table 2 show that planting sources and receivers 100 to 200 feet below the earth's surface or below the weathered layer can result in less attenuation. On the other hand, it is conventionally believed that planting sources or receivers a shallow distance below the surface would not be expected to have a large effect on the signal because this distance is a small fraction of the attentuation length. In addition, it is well known that the changes in the amplitude and phase of the seismic signal are small for distances that are small compared to a wavelength.
TABLE 2 ______________________________________ WAVELENGTHS AND ATTENUATION LENGTHS FOR THE WEATHERED LAYER v = 2,000 feet/second Q = 20 f (Hz) .lambda. (ft) L (ft) ______________________________________ 10 200 1300 20 100 600 40 50 300 80 25 160 100 20 130 ______________________________________
To increase the bandwidth of the signal, it is common to reduce the distance that the seismic wave travels in the weathered layer by planting explosive sources in shot holes drilled below the weathered layer or 100 to 200 feet below the ground surface. However, the acoustic detectors or geophones are typically disposed at or just below the surface (e.g. 0 to 0.5 feet) and are coupled to the surface by short spikes attached to the housing of the geophone. Similar methods are used for both conventional and high-resolution recording as described by Knapp and Steeples (Geophysics Vol. 51, 1986, p. 283-294).
In rare surveys, geophones have been placed 10 to 100 feet or more below the surface of the earth in order to couple the geophones to more competent material below the weathered layer. For example, Pullin, Mathews, and Hirshe (Geophysics: The Leading Edge of Exploration, Vol. 6, No. 12, 1987, P. 10-15) report on the planting of geophones 30 feet below a very absorbing Muskeg surface layer in Alberta, Canada. In addition, geophones have been planted in deep boreholes or drillholes in the soil layer as in the patent to Hawkins (U.S. Pat. No. 4,298,967). However, such surveys are expensive and time consuming because of the cost and effort in drilling a large number of deep holes and deploying geophones at these great depths. In addition, the geophones are rarely recoverable unless the holes are cased to prevent caving in of the holes.
Another factor that limits the bandwidth of seismic signals is the presence of noise. Noise is generally of two types: source generated noise and ambient noise. The largest type of source generated noise is low-frequency, horizontally propagating surface waves. These surface waves can be suppressed by the use of arrays. A receiver array typically consists of 6-12 geophones arranged at spaced intervals over 50 to 200 feet, and the signals from the individual geophones in the array are summed. The design of the optimum array lengths as well as other recording parameters is based on the results of a noise test conducted at the site prior to the seismic survey. However, arrays cannot be used when geophones are deeply buried because of the expense of drilling 6-12 holes per station.
Ambient noise generally has a broad frequency range. The high-frequency ambient noise interferes with high-frequency recording. It consists of noise from the surroundings. Sources of ambient noise include wind, traffic, livestock, machinery, etc. Deeply buried geophones can be used to shield the receivers from the ambient noise because the attenuating weathered layer will also attenuate noise propagating from the surface to the buried geophone.
There remains a need to increase the seismic bandwidth in high-resolution seismic surveys while avoiding the difficulties of deeply planted geophones.